1. Field of the Invention
This invention relates generally to multimode imaging systems using filter modules, and, more particularly, to such systems where the multimode characteristics can be changed by a relative motion of the filter module.
2. Description of the Related Art
A multimode imaging system is an imaging system that can capture information beyond the usual spatial information acquired by conventional imaging systems (e.g., those based on RGB Bayer patterns). For example, a multimode imaging system might acquire additional spectral information (multispectral and hyperspectral systems) to be used for spectral analysis or identification of substances. Multimode imaging systems might also capture information about the polarization of a scene, or even provide a higher dynamic range than that provided by the inherent capability of the detector array.
Acquiring this additional information can be difficult since most commercially available detector arrays spatially segment the incoming image into a two-dimensional image signal. Traditionally, the additional information of spectra, polarization or other modes was acquired by time multiplexing. For example, in spectral imaging applications, it might be desirable to acquire radiation from an object at different wavelengths of interest. The number of wavelength bands of interest may be between 5 and 20 for multispectral imagers and more than 20 for hyperspectral imagers. Traditional multispectral or hyperspectral imagers are based either on a filter wheel that contains wavelength filters that correspond to the wavelength bands of interest or on dispersive elements such as prisms or gratings. In case a filter wheel is used, at any one time, only one of the wavelength filters is positioned in the imaging path. The filter wheel rotates in order to switch from one wavelength filter to the next. Thus, the multispectral or hyperspectral imaging is implemented in a time multiplexed manner. However, the resulting systems can be large and complicated. In case dispersive elements are used to spatially separate different wavelengths, the light is typically dispersed along one dimension of the detector array. The other dimension is used to capture one spatial dimension of the object. However, it is difficult to also capture the second spatial dimension of the object. Sometimes, time multiplexing is introduced to capture the second spatial dimension, for example by scanning.
Recently, there has been an increased attention on acquiring multimode information of a scene simultaneously, or in a “single snapshot.” These single-snapshot systems multiplex the different mode signals onto different detector pixels in the detector array. That is, the multimode information is spatially multiplexed rather than time multiplexed.
Single snapshot multispectral imaging architectures can generally be categorized into two classes. One class uses dispersive elements, such as prisms or gratings, to spatially separate different wavelengths in combination with some beam splitting element, e.g. a prism or a mask. This architecture has the disadvantage that the dispersive element is typically applied to either a collimated beam or at an intermediate image plane. As a result, many of these systems are 4-f systems (four times the effective focal length of the optical system), which results in an optical system that is quite large and has limited field of view.
In the other class of single snapshot imagers, separate filters are attached to each detector in a manner similar to the RGB Bayer pattern found in conventional color imaging systems. That is, a color filter array is laid out on top of the detector array, so that individual filters (which will be referred to as micro-filters) each filter the light directed to individual detectors. The individual micro-filters are designed to implement the multimode imaging. For example, the filter array might include micro-filters with 20 different wavelength responses in order to implement multispectral imaging. One disadvantage of this class of systems is the increased cost and complexity of manufacturing. Because there is a one-to-one correspondence between micro-filters and detectors, and because the micro-filters are attached to the detectors, the micro-filters are the same size as the detectors, which is small. The many different small micro-filters must then be arranged into an array and aligned with the underlying detectors. Another disadvantage is the lack of flexibility. Once the micro-filter array is attached to the detector array, it is difficult to change the micro-filter array.
Thus, there is a need for improved multimode imaging systems.